1. Field of the Invention
The present invention relates to a noninvasive method and system for detecting various vascular conditions. More particularly, the present invention relates to a noninvasive method and system which utilizes an occlusive arm cuff as a plethysmograph to obtain arterial and endothelial data on a patient and to analyze the data for diagnosing and predicting various vascular conditions.
2. Description of the Related Art
An occlusive arm cuff capable of being filled with compressible air is routinely used to determine the arterial blood pressure. It has been proposed to also use the occlusive arm cuff as a plethysmograph, i.e., to measure the arterial volume, since it possesses compliance due to the fact that it is filled with compressible air. In theory, if the cuff compliance is known, the arterial pulsations, that are found in the cuff pressure, can be converted into the arterial volume pulse which can be used to measure arterial volume to render the occlusive arm cuff as a plethysmograph.
Unfortunately, cuff compliance is not a constant value. It is dependent on how tightly the arm cuff is wrapped on the subject""s arm and the current level of cuff pressure. This has prevented the use of the occlusive arm cuff as a plethysmograph.
The benefit of simultaneously measuring the arterial volume and pressure by means of the occlusive arm cuff is the ability to find arterial mechanics and wall properties. Such arterial information could then be obtained on patients as part of a routine physical exam. Furthermore, arterial wall properties can provide an early indication of the presence of a vascular disease process.
U.S. patent application Ser. No. 09/205,042 filed on Dec. 3, 1998 by Drzewiecki et al. and having a common assignee with the present application describes a method of calibrating the occlusive arm cuff to measure arterial volume to allow the arm cuff to be used as a plethysmograph. With reference to FIG. 1, there is shown a block diagram of the prior art occlusive arm cuff plethysmograph designated generally by reference numeral 100. The occlusive arm cuff plethysmograph includes a pump 102, a needle valve 104, a flow meter 106, a blood pressure cuff 108, and a pressure transducer 110. Pump 102 provides a constant known volume of gas per pump stroke over the relevant pressure range. The output signal from the transducer 110 is connected to an electronic amplifier 111, which in turn is connected to an analog-to-digital (A/D) converter and signal processing circuit 112. The A/D converter 112 outputs a first component 118 and a second component 120. The first component 118 is caused by compressed air in the arm cuff 108 by the pump 102. The second component 120 is caused by the patient""s arterial pulse. A motor speed control circuit 113 controls the frequency of the pump 102. A power supply 114 provides power to motor speed control circuit 113.
One typically begins the process by ascertaining the stroke volume of pump 102. This can be done by measuring the volume of a fluid (e.g., a gas, such as air) pumped by pump 102 over a period of time (e.g., ten seconds) divided by the number of strokes during that period of time. The fluid used to ascertain the stroke volume of pump 102 is typically the same fluid as that used to inflate the arm cuff 108 during use of the arm cuff 108. This process can take place simultaneously with data acquisition and is monitored throughout the data acquisition procedure by using the flow meter 106 and monitoring the signal received at the pressure transducer 110.
In short, the method entails applying a known volume change to the arm cuff using a periodic pump and subsequently requiring a skilled operator to calculate the cuff compliance. Since the pump frequency and the arterial pulse frequency differ, electronic filtering is used to separate each pulse from the cuff pressure. This method, thus provides continuous measurement of cuff compliance, thereby solving the cuff compliance problem. The method is referred to as occlusive arm cuff plethysmography.
In short, the occlusive arm cuff, acting as a plethysmograph, measures the arterial volume pulse as cuff pressure is decreased. In this manner the brachial artery compliance can be measured over the full range of cuff pressures. This allows the examination of all three states of the artery: collapse, buckling and distension.
To find the arterial compliance, one divides the arterial volume pulse by the arterial pulse pressure according to,       C    ⁡          (              P        t            )        =            Δ      ⁢              xe2x80x83            ⁢      V                      (                              P            systolic                    -                      P            diastolic                          )            ⁢              W        cuff            
Psystolic and Pdiastolic are the systolic and diastolic pressure obtained from the Korotkoff method. Pt is the arterial transmural pressure found from the difference between mean arterial pressure and mean cuff pressure,
Pt={overscore (Pa+L )}xe2x88x92{overscore (Pcuff+L )}
Wcuff is a constant that represents the cuff width. More precisely, it represents the actual volume of artery that is subjected to a uniform transmural pressure, Pt.
The lumen area can be found by integrating the compliance curve, from negative infinity to Pt,
A(Pt)=∫C(Pt)dPt
where negative infinity is approximated by the lowest transmural pressure measured or when the arterial lumen is completely collapsed. Since complete collapse corresponds with a lumen area equal to zero, the initial constant of this integration is zero as well. This analysis permits the determination of lumen area from noninvasive compliance measurements.
Other parameters can also be determined, such as the volume of a segment of an artery by integrating the arterial compliance, and blood flow through the artery by multiplying the derivative of the arterial volume with respect to time with the arterial compliance.
In one study, the occlusive arm cuff plethysmography was employed to derive brachial artery pressure (arterial pressure) versus lumen area curves (P-A curves) for several subjects. Drzewiecki et al., xe2x80x9cNoninvasive Measurement of the Human Brachial Artery Pressurexe2x80x94Area Relation In Collapse and Hypertension,xe2x80x9d Annals of Biomed. Eng., vol. 26, pages 965-974, 1998. A P-A curve for a normotensive and hypertensive subject are shown by FIG. 2. It is apparent from this data that the P-A curves are quite varied from subject to subject. First, the vessel is most compliant near zero pressure (i.e., when the blood pressure in the artery equals cuff pressure). Second, the brachial artery adapts to high blood pressure by increasing its lumen size. Hence, a single observation of the P-A curve in a given subject may be insufficient for diagnostic information. Accordingly, multiple observations of the same patient or perhaps the use of an intervention may be required.
The cross-section of normal, hypertensive and arteriosclerotic blood vessel changes are shown by FIG. 3. It is apparent that the vascular smooth muscle wall 2 of the hypertensive vessel becomes thicker and the lumen 4 becomes larger or remains the same. The lumen 4 of an arteriosclerotic vessel simply becomes narrower due to plaque 6 lining the wall 2. An important application of the occlusive arm cuff plethysmograph is to aid in the noninvasive detection of these important vessel changes, i.e., to measure smooth muscle or endothelial function. It is not certain how these conditions manifest themselves in the P-A curve or in a brachial artery pressure versus compliance curve (P-C curve) which represent a patient""s resting arterial mechanical function.
Due to the presence of arterial wall smooth muscle, the subject""s P-A and P-C curves may vary with muscle activity. Thus, the vascular smooth muscle is a part of the vascular function that must be examined. Prior research suggests that impaired muscle function, often termed as endothelial function, is predictive of atherosclerosis.
Hence, there is a need for additional research directed at measuring subjects known to possess various types of vascular conditions and to develop computer models that can predict or analyze changes in the P-A, P-C curves and other data. This will aid in diagnosis of the patient, since the specific alterations associated with a particular vascular condition can be recognized from the curve patterns. For example, research indicates that the arterial function is impaired in the presence of vascular disease, such as arteriosclerosis.
A need also exists in developing a cost effective process which employs a new instrument (based on this technology) that utilizes the conventional occlusive arm cuff to noninvasively measure the functional, geometrical, and mechanical condition of a patient""s arterial and endothelial function and to diagnose and predict various vascular conditions based on the computer models. Such a medical instrument can become part of routine patient physical examination. Currently, only more costly imaging techniques are available.
Additionally, a need also exists to incorporate the medical instrument into an existing automatic blood pressure monitor that employs an occlusive arm cuff to allow the blood pressure monitor to gather arterial and endothelial function information, where the medical instrument uses programmable instructions to determine cuff compliance of the arm cuff from physical laws of air flow and cuff pressure data. Hence, an operator of the instrument does not need to be mathematically skilled in calculating the cuff compliance.
The present invention provides a method and system for detecting various vascular conditions using an occlusive arm cuff plethysmograph. The system includes data acquisition hardware, including the occlusive arm cuff plethysmograph, for obtaining arterial and endothelial function data from a patient, processing means utilizing application or analysis software for analyzing the arterial and endothelial function data, and a database of computer models, such as brachial artery pressure versus lumen area curves (P-A curves) and brachial artery pressure versus compliance curves (P-C curves), developed by analyzing data for a plurality of subjects where their vascular conditions were known. The application software further diagnoses and predicts various vascular conditions pertaining to the patient by comparing or correlating the analyzed arterial and endothelial function data with the computer models stored within the database and presents the findings on a display.
In particular, the system of the present invention for determining at least one vascular condition includes the occlusive arm cuff plethysmograph having means for obtaining patient vascular data and means for outputting the patient vascular data; at least one database storing predetermined vascular data and a plurality of corresponding vascular conditions; and at least one processor.
The system further includes a plurality of programmable instructions, executed by the at least one processor, for determining compliance of the arm cuff from physical laws of air flow and cuff pressure data and for correlating the patient vascular data with the predetermined vascular data stored within the at least one database. The at least one processor then determines at least a portion of the predetermined vascular data which exhibits the maximum correlation with the patient vascular data. The at least one vascular condition is determined to be at least one of the plurality of corresponding vascular conditions which corresponds to a portion of the predetermined vascular data. The occlusive arm cuff plethysmograph further has means for connecting to a conventional blood pressure monitor, such that it uses the arm cuff provided by the blood pressure monitor.